Backstory: My bro-in-law had this 2G that he said"got some liquid/moisture in the bottom plug hole but not much. He reported it wouldn't make a call and the battery wouldn't hold a charge. He put it in a bowl of rice for a couple months and got a new 3G. He gave it to me to look at yesterday. So far it:

What works:
holds a charge just fine
wi-fi works fine
mail works fine
anything else I should test? besides cell service

What doesn't work:
Apps: they launch but close immediately upon their launch screens appearing, is this because there is no valid SIM in the phone?

I plugged in the phone with my working SIM from my 3GS and itunes wanted to update the software on the 2G(with my 3GS SIM installed). I said OK and it started to download the software update. I then took out my SIM and let it finish updating the software which it did perfectly.

I need to test the cell service on this phone. How do I got about doing that?
WHen I put my 3GS working SIM in the 2G it gives me the plug/itunes logo on the screen and iTunes wants to activate the phone. Do I just go ahead and let it activate the phone?? Will this mess up the activation on my 3GS?

Any advice is appreciated! Thanks!